Fix #197: Manual lacks a link back to PHPUnit homepage.

Adds a first link to the PHPUnit homepage to the primary navigation.

In exchange, removes the "PHPUnit" prefix from version links, to avoid visual clutter.

![phpunit-docs-nav-home](https://cloud.githubusercontent.com/assets/41992/3737758/112d3f14-173a-11e4-902b-207201423090.png)